Shadow DOM support

Current implementation misses methods to access element in a shadow DOM:
It is needed to have support for [GetElementShadowRoot](https://www.w3.org/TR/webdriver/#dfn-get-element-shadow-root) and the [FindElement](https://www.w3.org/TR/webdriver/#dfn-find-element-from-shadow-root) and [FindElements](https://www.w3.org/TR/webdriver/#dfn-find-elements-from-shadow-root)